Here's thread from another 'site, six years ago, with the same question you have (and several photos of such bolt). Scroll to post #5: K98k Forum

The rifle in that post is identified as S/N 45625, only 582 after yours.

An answer (from a moderator of said 'site) posted as being forwarded from Jon Speed (author of Mauser Smallbores):

Conveying Herr Speed's remarks:
"This Plunger unit appears to simply affect directly the firing pin movement as bolt is opened and closed. Its function is a little like the lock plunger on a 98 bolt sleeve unit from a mechanical point only. Its a Plunger activated by contact with parts of the system as the bolt rotates ! It would appear at thiis late stage 1933/34 that the Mauser folks were trying out one last concept for this generation that would soon phase out as the B series came on line."